Accumulation of charged defects at. local and bulk properties of

Transcription

1 Accumulation of charged defects at domain walls and its implication to local and bulk properties of polycrystalline lli BiFeO 3 Tadej Rojac Andreja Bencan, Hana Ursic, Bostjan Jancar, Gasper Tavcar, Maja Makarovic, Julian Walker, Barbara Malic Jozef Stefan Institute, Ljubljana, Slovenia Goran Drazic National Institute of Chemistry, Ljubljana, Slovenia Naonori Sakamoto Shizuoka University, Hamamatsu, Japan Dragan Damjanovic Swiss Federale Institute of Technology, Lausanne, Switzerland

2 Why are domain walls (DWs) in polycrystalline ferroelectrics so important? Polycrystalline ferroelectrics (high piezoelectricity sensors, actuators) Contributions to piezoelectric effect from multiple length scales Direct piezo Q = d F Bulk samples mm Piezoelectric charges Q Force (F) Lattice response (intrinsic; each domain) pm DW Displacements of DWs (extrinsic) nm 10 m Grain-to-grain elastic interactions P s P d m Force (F) Pramanick et al. 2011; Tsurumi et al. 1997; Fancher et al. 2017; Warren et al. 2006, Chandrasekaran et al. 2013, Zhang & Ren 2006.? Local electrically conductive paths m? Electric and elastic dipoles

3 DWs have their own properties! Electrical conduction at DWs first shown in BiFeO 3 thin films DW nanoelectronics (prototype DW memory) Conductive atomic-force microscopy (c-afm) c-afm map from surface of switched (110) epitaxial BiFeO 3 film 8 V V domain walls Seidel et al., Nat. Mater. 8, 229 (2009); Farokhipoor & Noheda, Phys. Rev. Lett. 107, (2011); Maksymovich et al., Nano. Lett. 11, 1906 (2011); Guyonnet et al., Adv. Mater. 23, 5377 (2011); Schroder et al., Adv. Funct. Mater. 22, 3936 (2012); Meier et al., Nat. Mater. 11, 284 (2012); Faraji et al., Appl. Phys. Lett. 110, (2017); Lindgren & Canalis, APL Mater. 5, (2017); Catalan et al., Rev. Mod. Phys. 84, 119 (2012); Sharma et al., Sci. Adv., in press. Conduction at DWs also observed in other thin-film and single crystal ferroelectrics: Pb(Zr,Ti)O 3, PbTiO 3, LiNbO 3, rare-earth manganites, KTiOPO 4.

4 Conductive DWs in polycrystalline BiFeO3 Aim: Explore DW conduction in polycrystalline BiFeO3 and see how it affects piezoelectric response PFM OP amplitude PFM OP phase c-afm Pz. d 33eff II (pa) (a.u.) m According to c c-afm AFM, DWs show enhanced electrical conductivity relative to domains V DC 20 V AC Current profile I (a.u.) Line analysis Current (a a.u.) c-afm c AFM m +33 V DC Rojac et al., Adv. Funct. Mater. 25, 2099 (2015) 2 Point analysis l i domain wall (2) EBSD + c-afm: +33 V DC domain (1) 0 Consistent with studies on BiFeO3 thin films! Distance ( m) Current vs. time Current (a.u.) V DC 3 m/s domain walls Time (s) 200 All types of DWs (71, 109, 180 ) a e conductive! are cond cti e!

5 ? How does local conductivity at DWs affect macroscopic piezoelectric response? macro local Converse piezo Conducting ferroelastic DWs new emerging g phenomena? x 3 = d 33 E 3 x mechanical strain d piezoelectric coefficient E electric field 3 longitudinal direction (along poling axis)

6 Conductive DWs in motion d 33 (pm/v) 70 error bar 14.6 E 0 -dependent E 0 -independent E dependent E 0 -independent E 0 (kv/cm) 30 E 0 (kv/cm) negative Frequency, f (Hz) Frequency, f (Hz) iezoelectric tan (/) P Electric field dependent low frequency response. Irreversible non 180 DW motion. Rojac et al., Adv. Funct. Mater. 25, 2099 (2015). Rojac et al., Appl. Phys. Lett. 109, (2016). DWs move irreversibly at low frequencies (depining) because conductivity is required for DWs to move (Postnikov et al., 1970). + Internal electric fields may change with time due to DW conductivity dielectric dispersion results in piezoelectric dispersion (D = d + E). = Nonlinear piezoelectric Maxwell Wagner mechanism (large effect: rel. increase of d 33 with E 0 at 0.2 Hz is ~40%).

7 Where does DW conductivity come from? Theoretical studies on DW conductivity: -BiFeO 3, Pb(Zr,Ti)O 3, LiNbO 3, BaTiO 3. - Reduction of bandgap at DWs, highsymmetry phase within DWs, charged DWs - Polarization and strain gradients at DWs accumulation of defects at DWs BiFeO 3, theory Néel 180 DW BiFeO DW: Succesive rotation of Ps by 71 and 109 creates variable P x across the DW; the resulting P charges should attract oppositely charged defects to the DW. PbTiO 3, theory Bandgap reduction at DWs predicted and experimentally confirmed in BiFeO 3 films BUT cannot explain, e.g., DW conductivity in PZT. 90 DW P x DW + BiFeO 3, experimental [100] Seidel et al., Nat. Mater. 8, 229 (2009); Lubk et al., PRB 80, (2009); Morozovska et al., PRB 86, (2012); Morozovska, Ferroelectrics 483, 3 (2012); Hong et al., PRB 77, (2008); Meyer & Vanderbilt, Phys. Rev. B 65, (2002); Choi et al., Nat. Mater. 9, 253 (2010); Chiu et al., Adv. Mater. 23, 1530 (2011); Catalan et al., Rev. Mod. Phys. 84, 119 (2012); Catalan & Scott, Adv. Mater. 21, 1 (2009). [001] [100] 109 DW Variable P x across 109 DW confirmed by STEM!

8 Atomic-scale STEM HAADF (Z-contrast); [010] Fe-displacement and c/a map [001] c [100] DW region [010] DW (109 ) 2nm 1.11 c/a a Jeol ARM 200 CF Analyses done on all types of DWs (109, 71, 180 ).

9 Evidence of defect accumulation at DWs ABF 1 nm HAADF Bi-column intensity map (normalized) EELS on DW EELS off DW on or off DW Fe L 3 O K Fe L 2 ΔE E (ev) Quantification (LeBeau & Stemmer) E (ev V) on DW off DW Fe 4+ Fe 3+ Fe 3+ Fe 4+ BiFeO 3 Fe 2.7+ Fe 2+ standards Fe oxidation state Tan et al., Ultramicr. 116, 24 (2012). Expe erimental Simulat ted 0 Int. 1 Bi Bi Bi/V Bi Bi Bi Bi Bi Bi/V Bi Bi Bi VBi O 2 Bi 3+ Fe 3+ Fe 4+ Bi/VBi QSTEM (K. Koch)

10 Which defects are expected in BiFeO 3? Different than in titanates (Pb(Zr,Ti)O ( 3, BaTiO 3 ) Fe 3+ can be both reduced (Fe 2+ ) and oxidized (Fe 4+ )! Fe 2+ Fe 4+ Older liter. (La,Sr)FeO 3 BFO-KBT, Ca-BFO BFO (unpublished) DFT p-type bulk conductivity in Cadoped BiFeO 3 can be reduced by annealing in N 2 (ionic regime)! Air Fe 4+ Wefring et al., Phys. Chem. Chem. Phys. 17, 9420 (2015); Maso & West, Chem. Mater. 24, 2127 (2012); Zhang et al., Appl. Phys. Lett. 96, (2010). In situ vs T curves during annealing in N 2 (BFO-KBT) N 2 p-type BiFeO I-V (room T) Fe Reversible by annealing in O 2! T ( C) j ( A/cm 2 ) air N E(kV/cm) AIR: DC = Ohm 1 m 1 N 2 : DC = Ohm 1 m 1 Courtesy of M. Morozov

11 Control of DW conductivity by p(o 2 ) 700 C air-sin ntered N 2 a c PFM b d c-afm Curre ent (a.u.) Current (a.u.) Current profiles 84% Distance ( m) 3% p(o )= atm p(o 2 )= atm DW conductivity DW conductivity p(o 2 ) p(o 2 ) e f O 2 Current (a.u.) 10 m +15 V DC Distance ( m) 88% Distance ( m) Statistical c-afm analysis on DWs per sample. p(o 2 )=1 atm Change of DW conductivity by annealing in different p(o 2 ) consistent with: - p-type bulk conductivity it - identified defects at DWs (Fe 4+ and Bi vacancies) Rojac et al., Nat. Mater. 16, 322 (2017).

12 Summary and conclusions c-afm +22 V DC I (pa) (a.u.) (pm/v) d E 0 -dependent E 0 -independent E 0 (kv/cm) Frequency, f (Hz) HAADF (Z-contrast); [010] [001] [100] EELS + DF image quantification DW Enhanced electrical conductivity at DWs found in polycrystalline samples. Local DW conductivity strongly affects macroscopic piezoelectric response via DW conductivity/dynamics coupling. 2 nm Implications: Control of type and concentration of defects essential not only for the bulk but also for the local conductivity. Results suggest pinning centers alternative to oxygen vacancies, opening up new possibilites for tailoring DW dynamics.

13 Acknowledgements Funding: - Slovenian Research Agency (program P2-0105, project J2-5483) - Centre of Excellence NAMASTE Thanks to: - David Žehelj - Andraž Bradeško -Tomaž Kos - Jana Cilensek, Silvo Drnovsek, Brigita Kmet - to all others that contributed